Mat,

Your first instinct is correct; a local engineer will have the best answer for you for the specific jurisdiction that you are dealing with.  I personally build in the highest seismic zone and with my experience your answer would be, it depends.

What it depends on is, depth of the lintel, span of the lintel, the size, grade, and number of tension bars being used for your lintel, where you are building, and what you would use as a mullion.

By code where I build we have to put 2 #5 60 KSI bars extending past the opening 2 feet in all directions for shear and crack control.  This is in most cases enough rebar to span 10 to 12 feet with at least 10 inches of concrete for the lintel.  So it typically does not matter what my mullion material is.  If you are using steel as a mullion this would count for strengthening the lintel, even a good Doug Fir or Yellow Pine would give you decent compressive strength, but Hem Fir, Spruce, Cedar and the like would be suspect and need an engineer’s approval here.

Hope this helps

ICF Contractor